In recent years, absorbent resins are used as one component material for sanitary cotton, disposable diapers, and other sanitary materials for absorbing body fluids and as a water retaining agent in agriculture and horticulture. As such absorbent resins, partially cross-linked polyacrylic acid, hydrolyzed starch-acrylonitrile graft polymer, neutralized starch-acrylic acid graft polymer, saponified vinyl acetate-acrylic ester copolymers, hydrolyzed acrylonitrile copolymers and acrylamide copolymers, and cross-linked products thereof have been known to the art, for example.
Generally, unaltered monomers persist in these absorbent resins. Particularly for the use of these absorbent resins in the fields of sanitary materials and foodstuffs, various attempts are made to decrease the contents of such unaltered monomers in the absorbent resins.
Heretofore, the attempt to decrease the residual monomer in the polymer has long been practised in the field of water-soluble polymers such as, for example, polymeric flocculants. The methods resorted to therefor are broadly classified, as noticed in JP-A-56-103207(1981), under the following three types.
(1) The methods relying on an increase in the amount of a polymerization initiator or on split addition of the polymerization initiator.
(2) The methods relying on extraction as with a solvent.
(3) The methods relying on transformation of the residual monomer to a derivative other than the polymer.
These techniques can be applied to the decrease of the residual monomer in the absorbent resin. The cases of applying methods of the type (1) to absorbent resins are reported in JP-A-56-72,005(1981) and JP-A-64-24,808(1989). When such a method is applied to an absorbent resin, a decrease in the molecular weight of the polymer, a decrease in the absorption capacity in consequence of an excessive self-cross-linking reaction, and an increase in the amount of a water-soluble component are recognized as in the case of applying the method to a water-soluble polymer. Though this method decreases the residual monomer, it impairs the properties of the absorbent resin. A method of the type (2) is reported in JP-A-1-292,003(1989). Though this method is effective in lowering the residual monomer content, it hardly deserves to be called a desirable way of decreasing the residual monomer because it necessitates use of a large amount of a solvent and consequently entails expenditure of a high cost for the recovery of used solvent. As methods of the type of (3) relying on the transformation of the residual monomer to a derivative other than the polymer, a method which contemplates effecting the decrease of the residual monomer in a water-soluble polymer by adding an amine or ammonia to the polymer thereby forming an adduct with the unaltered monomer (JP-A-33-2,646(1958)) and a method which comprises adding a hydrogen sulfite, a sulfite, or a pyrosulfite thereby forming an adduct with the unaltered monomer (JP-A-56-103,207(1981)).have been known to the art. Cases of applying such methods to absorbent resins are also reported JP-A-64-62-317(1989) and DE-A-3,724,709).
The follow-up experiments conducted by the present inventors, however, have revealed that the method disclosed in JP-A-64-62,317(1989) decreases the residual monomer only to an insufficient extent and the method disclosed in DE-A-3,724,709, though capable of effecting the decrease of the residual monomer, requires use of a large amount of water falling in the range of 50 to 500 parts by weight based on 100 parts by weight of the absorbent resin powder and consequently necessitates a step of drying and, therefore, not merely proves to be greatly disadvantageous economically but also brings about serious impairment of the various properties such as, for example, suction power and absorption capacity under pressure, inherently possessed by types of absorbent resins.
An object of this invention, therefore, is to provide an absorbent resin and a novel method for the production thereof.
Another object of this invention is to provide an absorbent resin excelling in absorption properties and possessing a low residual monomer content and a method for stable and efficient production thereof.